1. Field of the Invention
The present invention relates to an oven-controlled crystal oscillator (OCXO), the oven-controlled crystal oscillator being equipped with: a crystal unit or crystal element; a thermostatic oven which contains the crystal unit and keeps temperature of the crystal unit constant, and an oscillator circuit which uses the crystal unit.
2. Description of the Related Art
A crystal oscillator which incorporates, as an integral component, a quartz crystal unit and an oscillator circuit using the crystal unit outputs an oscillating signal of an accurate predetermined frequency simply when mounted on a circuit board or the like and supplied externally with a power supply voltage. The crystal oscillator is thus used widely as a frequency or time reference source in various electronic devices.
A vibration frequency of the crystal unit has high stability, but changes slightly with temperature due to frequency-temperature characteristics inherent to quartz. To deal with this, an oven-controlled crystal oscillator is available as a crystal oscillator which outputs a particularly accurate oscillation frequency, where the oven-controlled crystal oscillator includes a crystal unit or crystal element contained in a thermostatic oven or a constant temperature bath adapted to keep temperature of the crystal unit constant. Generally, the thermostatic oven is configured to be heated by an electric heater. Temperature in the thermostatic oven is detected by a temperature sensor installed in the thermostatic oven and detection results are fed back to a drive circuit of the heater to keep the temperature in the thermostatic oven constant. If the temperature of the thermostatic oven is set such as to minimize frequency changes due to temperature changes based on the frequency-temperature characteristics of the crystal unit, even if the temperature of the thermostatic oven varies minutely under the influence of ambient temperature, the oscillation frequency of the oven-controlled crystal oscillator is kept most stable. The vibration frequency of the crystal unit changes, for example, as a quadratic function or cubic function of temperature although this depends on the orientation in which a crystal blank (i.e., a vibrating piece) of the crystal unit is cut from a crystal of quartz. Consequently, the vibration frequency does not change with minute temperature changes around the temperature at a vertex of the function curve. This results in a zero temperature coefficient. Thus, this temperature is referred to as a zero temperature coefficient (ZTC) point. With the oven-controlled crystal oscillator, the temperature of the thermostatic oven is generally set at the ZTC point of the crystal unit.
FIG. 1 is a circuit diagram showing a configuration of an oven-controlled crystal oscillator according to a related art.
Thermostatic oven 10 houses crystal unit X and thermistor Th adapted to detect the temperature in thermostatic oven 10. Thermostatic oven 10 is designed to be heated by electric heater H, which is connected at one end to power supply terminal 11 and connected at the other end to a collector of power transistor 15 for driving. An emitter of transistor 15 is grounded. With the illustrated crystal oscillator, heater H is installed in thermostatic oven 10. Crystal unit X is electrically connected to oscillator circuit 13 which uses crystal unit X, and an oscillating signal is outputted from oscillator circuit 13 to output terminal 14. Here, oscillator circuit 13 is installed outside thermostatic oven 10, but may be installed in thermostatic oven 10. Power supply voltage Vcc1 is supplied externally to power supply terminal 11, but stabilized power supply circuit 12 is installed to generate more stable power supply voltage Vcc2 from power supply voltage Vcc1. Oscillator circuit 13 is supplied with power supply voltage Vcc2.
As thermistor Th, one with non-linear negative resistance-temperature characteristics is used. The negative resistance-temperature characteristics are resistance-temperature characteristics which have a negative resistance temperature coefficient. To drive heater H by feeding back detection results produced by thermistor Th, resistors R1 to R3 and differential amplifier 16 are installed and an output from the differential amplifier is connected to a base of power transistor 15. Thermistor Th is connected at one end to power supply voltage Vcc2, and at the other end to an inverting input terminal (−) of differential amplifier 16. Resistor R1 is installed between the inverting input terminal (−) and a ground point. Resistor R2 is installed between a non-inverting input terminal (+) of differential amplifier 16 and power supply voltage Vcc2 while resistor R3 is installed between the non-inverting input terminal (+) and ground point. Thus, thermistor Th, resistors R1 to R3, differential amplifier 16, and power transistor 15 make up a temperature control circuit of thermostatic oven 10. Except for thermistor Th, the elements making up the temperature control circuit are installed outside thermostatic oven 10 and affected by ambient temperature.
With this configuration, if resistors R1 to R3 are set such that a ratio between resistance value of thermistor Th at the ZTC point of crystal unit X and resistor R1 will coincide with a ratio between resistor R2 and resistor R3, differential amplifier 16 controls transistor 15 so as to reduce collector current of transistor 15 when the temperature in thermostatic oven 10 rises and consequently the resistance value of thermistor Th decreases, and conversely to increase collector current when the temperature in thermostatic oven 10 falls and consequently the resistance value of thermistor Th increases. Since the collector current of transistor 15 flows through heater H, heater current is controlled, after all, such that the temperature in thermostatic oven 10 will be kept at the ZTC point.
Actually, with the circuit shown in FIG. 1, whereas the heater current is zero when the temperature in thermostatic oven 10 is exactly at the ZTC point, the temperature in thermostatic oven 10 does not precisely reach the ZTC point and has some deviation from the ZTC point because there is some heat dissipation from thermostatic oven 10. To compensate for this deviation, it is conceivable to set resistors R1 to R3 such that a temperature slightly higher than the ZTC point will be a control target, but since an amount of heat dissipating from thermostatic oven 10 depends on the ambient temperature, the temperature of thermostatic oven 10 ends up being affected by the ambient temperature. Thus, to keep the temperature in the thermostatic oven at the ZTC point, JP2005-165630A proposes an oven-controlled crystal oscillator with a configuration in which a resistor corresponding to resistor R2 in the circuit shown in FIG. 1 is provided with linear positive resistance-temperature characteristics and arranged to change in resistance according to the ambient temperature such that the current flowing through heater H will increase with decreases in the ambient temperature.
JP2011-4382A discloses a technique for maintaining a predetermined temperature in the thermostatic oven by using a thermistor adapted to detect the ambient temperature as well as a thermistor adapted to detect the temperature in the thermostatic oven and performing open-loop control based on the ambient temperature as well as closed-loop control based on the temperature in the thermostatic oven.
The ZTC point of the crystal unit varies slightly from product to product even if crystal blanks cut in the same orientation are used and the vibration frequency is identical. Therefore, in order to produce an oven-controlled crystal oscillator with higher frequency accuracy, it is necessary observe the oscillation frequency and detect the ZTC point, with the crystal unit contained in the thermostatic oven while changing the temperature of the thermostatic oven by driving a heater adapted to heat the thermostatic oven, determine the target values of the resistors in the temperature control circuit according to the detected ZTC point, and adjust the resistors to the target values. For example, with the circuit configuration shown in FIG. 1, at least one of resistors R1 to R3 is adjusted. Actually, resistor R1 is adjusted in many cases. However, even if such adjustments are made, depending on the structure of the thermostatic oven and arrangement of the thermistor in the thermostatic oven, the thermistor temperature will not accurately match the temperature of the crystal unit. Also, since the amount of heat radiated to the surroundings from the thermostatic oven depends on the ambient temperature, the ZTC point determined based on the temperature detected by the thermistor, i.e., an apparent ZTC point itself, will vary with the ambient temperature. Since control is performed based on the temperature detected by the thermistor in the thermostatic oven, if the apparent ZTC point varies, the control cannot be performed properly. Even if the resistance values in the temperature control circuit are adjusted according to the ZTC point of the crystal unit, since the resistors themselves have resistance temperature coefficients and change their resistance values with the ambient temperature, it is also necessary to think about compensating for these changes.
After all, to construct a high-accuracy oven-controlled crystal oscillator, it is not enough to merely install a temperature sensor adapted to detect the ambient temperature in addition to a thermistor adapted to detect the temperature in the thermostatic oven, and it is necessary to give sufficient consideration to the configuration and adjustment method of the temperature control circuit.
When temperature control is performed using a thermistor, relationship between the ambient temperature and actual temperature in the thermostatic oven is, for example, roughly such that the temperature in the thermostatic oven rises with rises in the ambient temperature as indicated by a thick line in FIG. 2A because reduction in the amount of heat produced by the heater is insufficient compared with rises in the ambient temperature, or such that the temperature in the thermostatic oven falls with rises in the ambient temperature as indicated by a thick line in FIG. 2B because reduction in the amount of heat produced by the heater is excessive compared with rises in the ambient temperature. Which of the temperature characteristics shown in FIGS. 2A and 2B actually come into play depends on various factors including the structure and thermal environment of the thermostatic oven itself and its surroundings and the arrangement of the thermistor in the thermostatic oven. In either case, the temperature control circuit needs to be configured and adjusted so as to make compensation such that the temperature in the thermostatic oven will remain constant and coincide with a true ZTC point regardless of the ambient temperature as illustrated by broken lines in the FIGS. 2A and 2B.